Conventionally, there is known a multi-purpose rotary impact tool capable of changing over an impact mode in which bolts, nuts or screws are tightened by an impact mechanism having a hammer and an anvil and a drill driver mode in which drilling tasks such as pilot hole forming and screw tightening are performed while restraining the impact operation of the impact mechanism.
When drilling operations or other operations are carried out in the drill driver mode, it is necessary to reduce the operation speed to a value smaller than available in the impact mode in order to prevent occurrence of cracks or other trouble in a drill bit. In addition to the impact mechanism being restrained or released when changing over the respective modes, the speed reduction ratio is changed in each of the modes.
For example, Japanese Patent No. 3911905 (Patent Document 1) discloses a rotary impact tool that can be changed over between a low speed mode in which a planetary reduction mechanism operates and a high speed mode in which the planetary reduction mechanism does not operate. The high speed mode is used as an impact mode and the low speed mode as a drill driver mode.
Japanese Patent Laid-open Publication No. 2005-288682 (Patent Document 2) discloses a rotary impact tool in which a high speed mode can be changed between an impact mode and a drill driver mode depending on whether an impact mechanism is restrained or released. Thus, the drill driver mode can be changed between two different speed modes, i.e., a low speed mode for drilling purposes and a high speed mode for screw-tightening purposes.
However, the drill driver mode of the rotary impact tool disclosed in Patent Document 1 lies generally halfway between a high speed mode and a low speed mode available in a drill-driver-single-function rotary tool, such as an electric drill or the like. For that reason, the rotary impact tool disclosed in Patent Document 1 poses a problem in that the screw tightening speed thereof is slower than that available in the high speed mode of the single-function rotary tool. Another problem resides in that the rotary impact tool cannot perform an operation of drilling large-diameter holes as is available in the low speed mode of the single-function rotary tool.
With the rotary impact tool of Patent Document 2 in which the drill driver mode can be changed between two different speed modes, the performance in the low speed mode does not substantially differ from the performance in the low speed mode achieved by the rotary impact tool disclosed in Patent Document 1. Therefore, the rotary impact tool of Patent Document 2 cannot perform an operation of drilling large-diameter holes. Since the speed reduction ratio in the high speed drill driver mode is the same as that available in the impact mode, the rotary impact tool is apt to come into a locked state if the load thereof is increased during a heavy tightening operation or other operations. This makes it difficult to use the rotary impact tool.